Measuring the accuracy of high speed digitizers which operate in the subnanosecond time domain requires known time steps with known amplitude voltage waveform with rapid transitions. High speed transistors are used in balanced circuits for differential measurements which must be identified as to their common mode rejection of rapidly rising signals with accurately known rates of rise. Other circuits require linear rising waveforms of kilovolt amplitudes to produce high currents used to compress gaseous plasma discharges in fusion generation experiments.
The technology to produce these waveforms has been impractical because of excessive losses or limitations in the accuracy of the formation of the waveforms and limitations in the amplitudes of the waveforms.
A two-step waveform circuit in U.S. Pat. No. 3,629,740 to Merrill produces two equal amplitude steps but requires both 50-ohm and 100-ohm coaxial cable and requires a 50-ohm resistor. The supply of 100-ohm coaxial cable is limited and the impedance of 100-ohm coaxial cable is not necessarily exactly twice that of more readily available 50-ohm coaxial cable. The equality of the two steps produced by the Merrill circuit depends on the 100-ohm coaxial cable being equal to exactly twice that of the 50-ohm coaxial cable also used in the two-step waveform generating circuit. Skin effect losses in coaxial cable degrade the rise time of pulses traveling through the cable with the inner conductor, which has much less surface area for current flow than the outer conductor, causing most of the loss. The center conductor of 100-ohm coaxial cable has a diameter that ranges from approximately ⅓ to approximately ¼ the diameter of 50-ohm coaxial transmission line depending on the dielectric constant of the insulator supporting the center conductor. The two-step waveform in the Merrill circuit must travel through one 100-ohm transmission line with another 100-ohm stub transmission line used as a pulse forming element and with a 50-ohm stub transmission line element. The two stub transmission lines with different impedances have different high speed losses. The series 100-ohm transmission line also has different high speed losses. The different, high magnitude skin effect losses degrade the rise times of the two equal amplitude steps.